Thin film optical measurement system and method with calibrating ellipsometer

ABSTRACT

An optical measurement system for evaluating a reference sample that has at least a partially known composition. The optical measurement system includes a reference ellipsometer and at least one non-contact optical measurement device. The reference ellipsometer includes a light generator, an analyzer and a detector. The light generator generates a beam of quasimonochromatic light having a known wavelength and a known polarization for interacting with the reference sample. The beam is directed at a non-normal angle of incidence relative to the reference sample to interact with the reference sample. The analyzer creates interference between the S and P polarized components in the light beam after the light beam has interacted with reference sample. The detector measures the intensity of the light beam after it has passed through the analyzer. A processor determines the polarization state of the light beam entering the analyzer from the intensity measured by the detector, and determines an optical property of the reference sample based upon the determined polarization state, the known wavelength of light from the light generator and the composition of the reference sample. The processor also operates the optical measurement device to measure an optical parameter of the reference sample. The processor calibrates the optical measurement device by comparing the measured optical parameter from the optical measurement device to the determined optical property from the reference ellipsometer.

This is a continuation of application Ser. No. 08/890,697, filed Jul.11, 1997, now U.S. Pat. No. 5,798,837.

FIELD OF THE INVENTION

The present invention relates to optical analyzers, and moreparticularly to a thin film optical measurement system having acalibrating ellipsometer.

BACKGROUND OF THE INVENTION

There is considerable interest in developing systems for accuratelymeasuring the thickness and/or composition of thin films. The need isparticularly acute in the semiconductor manufacturing industry where thethickness of these thin film oxide layers on semiconductor substrates ismeasured. To be useful, the measurement system must be able to determinethe thickness and/or composition of films with a high degree ofaccuracy. The preferred measurement systems rely on non-contact, opticalmeasurement techniques, which can be performed during the semiconductormanufacturing process without damaging the wafer sample. Such opticalmeasurement techniques include directing a probe beam to the sample, andmeasuring one or more optical parameters of the reflected probe beam.

In order to increase measurement accuracy and to gain additionalinformation about the target sample, multiple optical measuring devicesare incorporated into a single composite optical measurement system. Forexample, the present assignee has marketed a product called OPTI-PROBE,which incorporates several optical measurement devices, including a BeamProfile Reflectometer (BPR), a Beam Profile Ellipsometer (BPE), and aBroadband Reflective Spectrometer (BRS). Each of these devices measuresparameters of optical beams reflected by, or transmitted through, thetarget sample. The BPR and BPE devices utilize technology described inU.S. Pat. Nos. 4,999,014 and 5,181,080 respectively, which areincorporated herein by reference.

The composite measurement system mentioned above combines the measuredresults of each of the measurement devices to precisely derive thethickness and composition of the thin film and substrate of the targetsample. However, the accuracy of the measured results depends uponprecise initial and periodic calibration of the measurement devices inthe optical measurement system. Further, recently developed measurementdevices have increased sensitivity to more accurately measure thinnerfilms and provide additional information about film and substratecomposition. These newer systems require very accurate initialcalibration. Further, heat, contamination, optical damage, alignment,etc., that can occur over time in optical measurement devices, affectthe accuracy of the measured results. Therefore, periodic calibration isnecessary to maintain the accuracy of the composite optical measurementsystem.

It is known to calibrate optical measurement devices by providing areference sample having a known substrate, with a thin film thereonhaving a known composition and thickness. The reference sample is placedin the measurement system, and each optical measurement device measuresthe optical parameters of the reference sample, and is calibrated usingthe results from the reference sample and comparing them to the knownfilm thickness and composition. A common reference sample is a "nativeoxide" reference sample, which is a silicon substrate with an oxidelayer formed thereon having a known thickness (about 20 angstroms).After fabrication, the reference sample is kept in a non-oxygenenvironment to minimize any further oxidation and contamination thatchanges the thickness of the reference sample film away from the knownthickness, and thus reduces the effectiveness of the reference samplefor accurate calibration. The same reference sample can be reused toperiodically calibrate the measurement system. However, if and when theamount of oxidation or contamination of the reference sample changes thefilm thickness significantly from the known thickness, the referencesample must be discarded.

For many optical measurement devices, reference samples with knownthicknesses have been effective for system calibration. Oxidation andcontamination that routinely occurs over time with reference samples istolerable because the film thickness change resulting from theoxidation/contamination is relatively insignificant compared to theoverall thickness of the film (around 100 angstroms). However, newultra-sensitive optical measurement systems have been recently developedthat can measure film layers with thicknesses less than 10 angstroms.These systems require reference samples having film thicknesses on theorder of 20 angstroms for accurate calibration. For such thin filmreference samples, however, the changes in film layer thicknessresulting from even minimal oxidation or contamination are significantcompared to the overall "known" film layer thickness, and result insignificant calibration error. Therefore, it is extremely difficult, ifnot impossible, to provide a native oxide reference sample with a knownthickness that is stable enough over time to be used for periodiccalibration of ultra-sensitive optical measurement systems.

There is a need for a calibration method for ultra-sensitive opticalmeasurement devices that can utilize a reference sample that does nothave a stable or known film thickness.

SUMMARY OF THE INVENTION

The present invention is a thin film optical measurement system with awavelength stable calibration ellipsometer that precisely determines thethickness of a film on a reference sample. The measured results from thecalibration ellipsometer are used to calibrate other optical measurementdevices in the thin film optical measurement system. By not having tosupply a reference sample with a predetermined known film thickness, areference sample having a film with a known composition can berepeatedly used to calibrate ultra-sensitive optical measurementdevices, even if oxidation or contamination of the reference samplechanges the thickness of the film over time.

The calibration reference ellipsometer uses a reference sample that hasat least a partially known composition to calibrate at least one othernon-contact optical measurement device. The reference ellipsometerincludes a light generator that generates a quasi-monochromatic beam oflight having a known wavelength and a known polarization for interactingwith the reference sample. The beam is directed at a non-normal angle ofincidence relative to the reference sample to interact with thereference sample. An analyzer creates interference between S and Ppolarized components in the light beam after the light beam hasinteracted with reference sample. A detector measures the intensity ofthe light after the beam has passed through the analyzer. A processordetermines the polarization state of the light beam entering theanalyzer from the intensity measured by the detector. The processor thendetermines optical properties of the reference sample based upon thedetermined polarization state, the known wavelength of light from thelight generator and the at least partially known composition of thereference sample. Wherein the processor operates at least one othernon-contact optical measurement device that measures an opticalparameter of the reference sample. The processor calibrates the otheroptical measurement device by comparing the measured optical parameterfrom the other optical measurement device to the determined opticalproperty from the reference ellipsometer.

Other aspects and features of the present invention will become apparentby a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a composite optical measurement system with thecalibration ellipsometer of the present invention.

FIG. 2 is a side cross-sectional view of the reflective lens used withthe present invention.

FIG. 3 is a plan view of an alternate embodiment of the light source forthe calibration ellipsometer of the present invention.

FIG. 4 is a plan view of the composite optical measurement system withmultiple compensators in the calibration ellipsometer of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a composite thin film optical measurementsystem 1 having a wavelength stable reference ellipsometer 2 that isused, in conjunction with a reference sample 4 having a substrate 6 andthin film 8 with known compositions, to calibrate non-contact opticalmeasurement devices contained in the composite thin film opticalmeasurement system 1.

FIG. 1 illustrates the composite optical measurement system 1 that hasbeen developed by the present assignees, which includes five differentnon-contact optical measurement devices and the reference ellipsometer 2of the present invention.

Composite optical measurement system 1 includes a Beam ProfileEllipsometer (BPE) 10, a Beam Profile Reflectometer (BPR) 12, aBroadband Reflective Spectrometer (BRS) 14, a Deep Ultra VioletReflective Spectrometer (DUV) 16, and a Broadband SpectroscopicEllipsometer (BSE) 18. These five optical measurement devices utilize asfew as two optical sources: laser 20 and white light source 22. Laser 20generates a probe beam 24, and white light source 22 generates probebeam 26 (which is collimated by lens 28 and directed along the same pathas probe beam 24 by mirror 29). Laser 20 ideally is a solid state laserdiode from Toshiba Corp. which emits a linearly polarized 3 mW beam at673 nm. White light source 22 is ideally a deuterium-tungsten lamp thatproduces a 200 mW polychromatic beam that covers a spectrum of 200 nm to800 nm. The probe beams 24/26 are reflected by mirror 30, and passthrough mirror 42 to sample 4.

The probe beams 24/26 are focused onto the surface of the sample with alens 32 or lens 33. In the preferred embodiment, two lenses 32/33 aremounted in a turret (not shown) and are alternatively movable into thepath of probe beams 24/26. Lens 32 is a spherical, microscope objectivelens with a high numerical aperture (on the order of 0.90 NA) to createa large spread of angles of incidence with respect to the samplesurface, and to create a spot size of about one micron in diameter. Lens33 is illustrated in FIG. 2, and is a reflective lens having a lowernumerical aperture (on the order of 0.4 NA) and capable of focusing deepUV light to a spot size of about 10-15 microns.

Beam profile ellipsometry (BPE) is discussed in U.S. Pat. No. 5,181,080,issued Jan. 19, 1993, which is commonly owned by the present assigneeand is incorporated herein by reference. BPE 10 includes a quarter waveplate 34, polarizer 36, lens 38 and a quad detector 40. In operation,linearly polarized probe beam 24 is focused onto sample 4 by lens 32.Light reflected from the sample surface passes up through lens 32,through mirrors 42, 30 and 44, and directed into BPE 10 by mirror 46.The position of the rays within the reflected probe beam correspond tospecific angles of incidence with respect to the sample's surface.Quarter-wave plate 34 retards the phase of one of the polarizationstates of the beam by 90 degrees. Linear polarizer 36 causes the twopolarization states of the beam to interfere with each other. Formaximum signal, the axis of the polarizer 36 should be oriented at anangle of 45 degrees with respect to the fast and slow axis of thequarter-wave plate 34. Detector 40 is a quad-cell detector with fourradially disposed quadrants that each intercept one quarter of the probebeam and generate a separate output signal proportional to the power ofthe portion of the probe beam striking that quadrant. The output signalsfrom each quadrant are sent to a processor 48. As discussed in the U.S.Pat. No. 5,181,080, by monitoring the change in the polarization stateof the beam, ellipsometric information, such as ψ and Δ, can bedetermined. To determine this information, the processor 48 takes thedifference between the sums of the output signals of diametricallyopposed quadrants, a value which varies linearly with film thickness forvery thin films.

Beam profile reflectometry (BPR) is discussed in U.S. Pat. No.4,999,014, issued on Mar. 12, 1991, which is commonly owned by thepresent assignee and is incorporated herein by reference. BPR 12includes a lens 50, beam splitter 52 and two linear detector arrays 54and 56 to measure the reflectance of the sample. In operation, linearlypolarized probe beam 24 is focused onto sample 4 by lens 32, withvarious rays within the beam striking the sample surface at a range ofangles of incidence. Light reflected from the sample surface passes upthrough lens 32, through mirrors 42 and 30, and directed into BPR 12 bymirror 44. The position of the rays within the reflected probe beamcorrespond to specific angles of incidence with respect to the sample'ssurface. Lens 50 spatially spreads the beam two-dimensionally. Beamsplitter 52 separates the S and P components of the beam, and detectorarrays 54 and 56 are oriented orthogonal to each other to isolateinformation about S and P polarized light. The higher angles ofincidence rays will fall closer to the opposed ends of the arrays. Theoutput from each element in the diode arrays will correspond todifferent angles of incidence. Detector arrays 54/56 measure theintensity across the reflected probe beam as a function of the angle ofincidence with respect to the sample surface. The processor 48 receivesthe output of the detector arrays 54/56, and derives the thickness andrefractive index of the thin film layer 8 based on these angulardependent intensity measurements by utilizing various types of modelingalgorithms. Optimization routines which use iterative processes such asleast square fitting routines are typically employed. One example ofthis type of optimization routine is described in "MultiparameterMeasurements of Thin Films Using Beam-Profile Reflectivity," Fanton, et.al., Journal of Applied Physics, Vol. 73, No. 11, p.7035, 1993.

Broadband reflective spectrometer (BRS) 14 simultaneously probes thesample 4 with multiple wavelengths of light. BRS 14 uses lens 32 andincludes a broadband spectrometer 58 which can be of any typecommonlyknown and used in the prior art. The spectrometer 58 shown inFIG. 1 includes a lens 60, aperture 62, dispersive element 64 anddetector array 66. During operation, probe beam 26 from white lightsource 22 is focused onto sample 4 by lens 32. Light reflected from thesurface of the sample passes up through lens 32, and is directed bymirror 42 (through mirror 84) to spectrometer 58. The lens 60 focusesthe probe beam through aperture 62, which defmes a spot in the field ofview on the sample surface to analyze. Dispersive element 64, such as adiffraction grating, prism or holographic plate, angularly disperses thebeam as a function of wavelength to individual detector elementscontained in the detector array 66. The different detector elementsmeasure the optical intensities of the different wavelengths of lightcontained in the probe beam, preferably simultaneously. Alternately,detector 66 can be a CCD camera, or a photomultiplier with suitablydispersive or otherwise wavelength selective optics. It should be notedthat a monochrometer could be used to measure the different wavelengthsserially (one wavelength at a time) using a single detector element.Further, dispersive element 64 can also be configured to disperse thelight as a function of wavelength in one direction, and as a function ofthe angle of incidence with respect to the sample surface in anorthogonal direction, so that simultaneous measurements as a function ofboth wavelength and angle of incidence are possible. Processor 48processes the intensity information measured by the detector array 66.

Deep ultra violet reflective spectrometry (DUV) simultaneously probesthe sample with multiple wavelengths of ultra-violet light. DUV 16 usesthe same spectrometer 58 to analyze probe beam 26 as BRS 14, except thatDUV 16 uses the reflective lens 33 instead of focusing lens 32. Tooperate DUV 16, the turret containing lenses 32/33 is rotated so thatreflective lens 33 is aligned in probe beam 26. The reflective lens 33is necessary because solid objective lenses cannot sufficiently focusthe UV light onto the sample.

Broadband spectroscopic ellipsometry (BSE) is discussed in pending U.S.patent application Ser. No. 08/685,606, filed on Jul. 24, 1996, which iscommonly owned by the present assignee and is incorporated herein byreference. BSE (18) includes a polarizer 70, focusing mirror 72,collimating mirror 74, rotating compensator 76, and analyzer 80. Inoperation, mirror 82 directs at least part of probe beam 26 to polarizer70, which creates a known polarization state for the probe beam,preferably a linear polarization. Mirror 72 focuses the beam onto thesample surface at an oblique angle, ideally on the order of 70 degreesto the normal of the sample surface. Based upon well known ellipsometricprinciples, the reflected beam will generally have a mixed linear andcircular polarization state after interacting with the sample, basedupon the composition and thickness of the sample's film 8 and substrate6. The reflected beam is collimated by mirror 74, which directs the beamto the rotating compensator 76. Compensator 76 introduces a relativephase delay δ (phase retardation) between a pair of mutually orthogonalpolarized optical beam components. Compensator 8 is rotated at anangular velocity ω about an axis substantially parallel to thepropagation direction of the beam, preferably by an electric motor 78.Analyzer 80, preferably another linear polarizer, mixes the polarizationstates incident on it. By measuring the light transmitted by analyzer80, the polarization state of the reflected probe beam can bedetermined. Mirror 84 directs the beam to spectrometer 58, whichsimultaneously measures the intensities of the different wavelengths oflight in the reflected probe beam that pass through thecompensator/analyzer combination. Processor 48 receives the output ofthe detector 66, and processes the intensity information measured by thedetector 66 as a function of wavelength and as a function of the azimuth(rotational) angle of the compensator 76 about its axis of rotation, tosolve the ellipsometric values ψ and Δ as described in U.S. patentapplication Ser. No. 08/685,606.

Detector/camera 86 is positioned above mirror 46, and can be used toview reflected beams off of the sample 4 for alignment and focuspurposes.

In order to calibrate BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18, thecomposite optical measurement system 1 includes the wavelength stablecalibration reference ellipsometer 2 used in conjunction with areference sample 4. Ellipsometer 2 includes a light source 90, polarizer92, lenses 94 and 96, rotating compensator 98, analyzer 102 and detector104.

Light source 90 produces a quasi-monochromatic probe beam 106 having aknown stable wavelength and stable intensity. This can be donepassively, where light source 90 generates a very stable outputwavelength which does not vary over time (i.e. varies less than 1%).Examples of passively stable light sources are a helium-neon laser, orother gas discharge laser systems. Alternately, a non-passive system canbe used as illustrated in FIG. 3 where the light source 90 includes alight generator 91 that produces light having a wavelength that is notprecisely known or stable over time, and a monochrometer 93 thatprecisely measures the wavelength of light produced by light generator91. Examples of such light generators include laser diodes, orpolychromatic light sources used in conjunction with a color filter suchas a grating. In either case, the wavelength of beam 106, which is aknown constant or measured by monochrometer 93, is provided to processor48 so that ellipsometer 2 can accurately calibrate the opticalmeasurement devices in system 1.

The beam 106 interacts with polarizer 92 to create a known polarizationstate. In the preferred embodiment, polarizer 92 is a linear polarizermade from a quartz Rochon prism, but in general the polarization doesnot necessarily have to be linear, nor even complete. Polarizer 92 canalso be made from calcite. The azimuth angle of polarizer 92 is orientedso that the plane of the electric vector associated with the linearlypolarized beam exiting from the polarizer 92 is at a known angle withrespect to the plane of incidence (defined by the propagation directionof the beam 106 and the normal to the surface of sample 4). The azimuthangle is preferably selected to be on the order of 30 degrees becausethe sensitivity is optimized when the reflected intensities of the P andS polarized components are approximately balanced. It should be notedthat polarizer 92 can be omitted if the light source 90 emits light withthe desired known polarization state.

The beam 106 is focused onto the sample 4 by lens 94 at an obliqueangle. For calibration purposes, reference sample 4 ideally consists ofa thin oxide layer 8 having a thickness d, formed on a silicon substrate6. However, in general, the sample 4 can be any appropriate substrate ofknown composition, including a bare silicon wafer, and silicon wafersubstrates having one or more thin films thereon. The thickness d of thelayer 8 need not be known, or be consistent between periodiccalibrations. The useful light from probe beam 106 is the lightreflected by the sample 4 symmetrically to the incident beam about thenormal to the sample surface. It is noted however that the polarizationstate of nonspecularly scattered radiation can be determined by themethod of the present invention as well. The beam 106 is ideallyincident on sample 4 at an angle on the order of 70 degrees to thenormal of the sample surface because sensitivity to sample properties ismaximized in the vicinity of the Brewster or pseudo-Brewster angle of amaterial. Based upon well known ellipsometric principles, the reflectedbeam will generally have a mixed linear and circular polarization stateafter interacting with the sample, as compared to the linearpolarization state of the incoming beam. Lens 96 collimates beam 106after its reflection off of the sample 4. The beam 106 then passesthrough the rotating compensator (retarder) 98, which introduces arelative phase delay δ (phase retardation) between a pair of mutuallyorthogonal polarized optical beam components. The amount of phaseretardation is a function of the wavelength, the dispersioncharacteristics of the material used to form the compensator, and thethickness of the compensator. Compensator 98 is rotated at an angularvelocity ω about an axis substantially parallel to the propagationdirection of beam 106, preferably by an electric motor 100. Compensator98 can be any conventional wave-plate compensator, for example thosemade of crystal quartz. The thickness and material of the compensator 98are selected such that a desired phase retardation of the beam isinduced. In the preferred embodiment, compensator 98 is a bi-platecompensator constructed of two parallel plates of anisotropic (usuallybirefringent) material, such as quartz crystals of opposite handedness,where the fast axes of the two plates are perpendicular to each otherand the thicknesses are nearly equal, differing only by enough torealize a net first-order retardation for the wavelength produced by thelight source 90.

Beam 106 then interacts with analyzer 102, which serves to mix thepolarization states incident on it. In this embodiment, analyzer 102 isanother linear polarizer, preferably oriented at an azimuth angle of 45degrees relative to the plane of incidence. However, any optical devicethat serves to appropriately mix the incoming polarization states can beused as an analyzer. The analyzer 102 is preferably a quartz Rochon orWollaston prism. The rotating compensator 98 changes the polarizationstate of the beam as it rotates such that the light transmitted byanalyzer 102 is characterized by: ##EQU1## where E_(x) and E_(y) are theprojections of the incident electric field vector parallel andperpendicular, respectively, to the transmission axis of the analyzer, δis the phase retardation of the compensator, and ω is the angularrotational frequency of the compensator.

For linearly polarized light reflected at non-normal incidence from thespecular sample, we have

    E.sub.x =r.sub.p cosP                                      (3a)

    E.sub.y =r.sub.s sinp                                      (3b)

where P is the azimuth angle of the incident light with respect to theplane of incidence. The coefficients a_(o), b₂, a₄, and b₄ can becombined in various ways to determine the complex reflectance ratio:

    r.sub.p /r.sub.s =tanψe.sup.iΔ.                  (4)

It should be noted that the compensator 98 can be located either betweenthe sample 4 and the analyzer 102 (as shown in FIG. 1), or between thesample 4 and the polarizer 92, with appropriate and well known minorchanges to the equations. It should also be noted that polarizer 70,lenses 94/96, compensator 98 and polarizer 102 are all optimized intheir construction for the specific wavelength of light produced bylight source 90, which maximizes the accuracy of ellipsometer 2.

Beam 106 then enters detector 104, which measures the intensity of thebeam passing through the compensator/analyzer combination. The processor48 processes the intensity information measured by the detector 104 todetermine the polarization state of the light after interacting with theanalyzer, and therefore the ellipsometric parameters of the sample. Thisinformation processing includes measuring beam intensity as a functionof the azimuth (rotational) angle of the compensator about its axis ofrotation. This measurement of intensity as a function of compensatorrotational angle is effectively a measurement of the intensity of beam106 as a function of time, since the compensator angular velocity isusually known and a constant.

By knowing the composition of reference sample 4, and by knowing theexact wavelength of light generated by light source 90, the opticalproperties of reference sample 4, such as film thickness d, refractiveindex and extinction coefficients, etc., can be determined byellipsometer 2. If the film is very thin, such as less than or equal toabout 20 angstroms, the thickness d can be found to first order in d/λby solving ##EQU2## where ##EQU3## which is the value of ρ=tanΨe^(i)Δfor d=0. Here, λ=wavelength of light; and ε_(s), ε_(o) and ε_(a) are thedielectric functions of the substrate, thin oxide film, and ambient,respectively, and θ is the angle of incidence.

If the film thickness d is not small, then it can be obtained by solvingthe equations

    ρ=r.sub.p /r.sub.s, where                              (8) ##EQU4## and where ##EQU5## and in general

    n.sub.j⊥ =(ε.sub.j -ε.sub.a sin.sup.2 θ).sup.1/2,(17)

where j is s or a. These equations generally have to be solvednumerically for d and n_(o) simultaneously, using ε_(s), ε_(a), λ, andθ, which are known.

Once the thickness d of film 8 has been determined by ellipsometer 2,then the same sample 4 is probed by the other optical measurementdevices BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18 which measure variousoptical parameters of the sample 4. Processor 48 then calibrates theprocessing variables used to analyze the results from these opticalmeasurement devices so that they produce accurate results. For each ofthese measurement devices, there are system variables that affect themeasured data and need to be accounted for before an accuratemeasurement of other samples can be made. In the case of BPE 10, themost significant variable system parameter is the phase shift thatoccurs due to the optical elements along the BPE optical path.Environmental changes to these optical elements result in an overalldrift in the ellipsometric parameter Δ, which then translates into asample thickness drift calculated by the processor 48 from BPE 10. Usingthe measured optical parameters of BPE 10 on reference sample 4, andusing Equation 5 and the thickness of film 8 as determined fromcalibration ellipsometer 2, the processor 48 calibrates BPE 10 byderiving a phase offset which is applied to measured results from BPE 10for other samples, thereby establishing an accurate BPE measurement. ForBSE 18, multiple phase offsets are derived for multiple wavelengths inthe measured spectrum.

For the remaining measurement devices, BPR 12, BRS 14 and DUV 16, themeasured reflectances can also be affected by environmental changes tothe optical elements in the beam paths. Therefore, the reflectancesR_(ref) measured by BPR 12, BRS 14 and DUV 16 for the reference sample 4are used, in combination with the measurements by ellipsometer 2, tocalibrate these systems. Equations 9-17 are used to calculate theabsolute reflectances R^(c) _(ref) of reference sample 4 from themeasured results of ellipsometer 2. All measurements by the BPR/BRS/DUVdevices of reflectance (R_(s)) for any other sample are then scaled byprocessor 48 using the normalizing factor in equation 18 below to resultin accurate reflectances R derived from the BPR, BRS and DUV devices:

    R=R.sub.s (R.sup.c.sub.ref /R.sub.ref)                     (18)

In the above described calibration techniques, all system variablesaffecting phase and intensity are determined and compensated for usingthe phase offset and reflectance normalizing factor discussed above,thus rendering the optical measurements made by these calibrated opticalmeasurement devices absolute.

The above described calibration techniques are based largely uponcalibration using the derived thickness d of the thin film. However,calibration using ellipsometer 2 can be based upon any of the opticalproperties of the reference sample that are measurable or determinableby ellipsometer 2 and/or are otherwise known, whether the sample has asingle film thereon, has multiple films thereon, or even has no filmthereon (bare sample).

The advantage of the present invention is that a reference sample havingno thin film thereon, or having thin film thereon with an unknownthickness which may even vary slowly over time, can be repeatedly usedto accurately calibrate ultra-sensitive optical measurement devices.

The output of light source 90 can also be used to calibrate thewavelength measurements made by spectrometer 58. The sample 4 can betipped, or replaced by a tipped mirror, to direct beam 106 up to mirror42 and to dispersion element 64. By knowing the exact wavelength oflight produced by light source 90, processor 48 can calibrate the outputof detector 66 by determining which pixel(s) corresponds to thatwavelength of light.

It should be noted that the calibrating ellipsometer 2 of the presentinvention is not limited to the specific rotating compensatorellipsometer configuration discussed above. The scope of the presentinvention includes any ellipsometer configuration in conjunction withthe light source 90 (having a known wavelength) that measures thepolarization state of the beam after interaction with the sample andprovides the necessary information about sample 4 for calibratingnon-contact optical measurement devices.

For example, another ellipsometric configuration is to rotate polarizer92 or analyzer 100 with motor 100, instead of rotating the compensator98. The above calculations for solving for thickness d still apply.

In addition, null ellipsometry, which uses the same elements asellipsometer 2 of FIG. 1, can be used to determine the film thickness dfor calibration purposes. The ellipsometric information is derived byaligning the azimuthal angles of these elements until a null or minimumlevel intensity is measured by the detector 104. In the preferred nullellipsometry embodiment, polarizers 92 and 102 are linear polarizers,and compensator 98 is a quarter-wave plate. Compensator 98 is aligned sothat its fast axis is at an azimuthal angle of 45 degrees relative tothe plane of incidence of the sample 4. Polarizer 92 has a transmissionaxis that forms an azimuthal angle P relative to the plane of incidence,and polarizer 102 has a transmission axis that forms an azimuthal angleA relative to the plane of incidence. Polarizers 92 and 102 are rotatedabout beam 106 such that the light is completely extinguished(minimized) by the analyzer 102. In general, there are two polarizer92/102 orientations (P₁, A₁) and (P₂, A₂) that satisfy this conditionand extinguish the light. With the compensator inducing a 90 degreephase shift and oriented with an azimuthal angle at 45 degree relativeto the plane of incidence, we have:

    P.sub.2 =P.sub.1 ±π/2                                (19)

    A.sub.2 =-A.sub.1                                          (20)

    ψ=A.sub.1 ≧0                                    (21)

(where A₁ is the condition for which A is positive).

    Δ=2P.sub.1 +π/2                                   (22)

which, when combined with equations 5-10, allows the processor to solvefor thickness d.

Null ellipsometry is very accurate because the results depend entirelyentirely on the measurement of mechanical angles, and are independent ofintensity. Null ellipsometry is further discussed by R. M. A. Azzam andN. M. Bashara, in Ellipsometry and Polarized Light (North-Holland,Amsterdam, 1977); and by D. E. Aspnes, in Optical Properties of Solids:New Developments, ed. B. O. Seraphin (North-Holland, Amsterdam, 1976),p. 799.

It is also conceivable to omit compensator 98 from ellipsometer 2, anduse motor 100 to rotate polarizer 92 or analyzer 102. Either thepolarizer 92 or the analyzer 102 is rotated so that the detector signalcan be used to accurately measure the linear polarization component ofthe reflected beam. Then, the circularly polarized component is inferredby assuming that the beam is totally polarized, and what is not linearlypolarized must be circularly polarized. Such an ellipsometer, commonlycalled a rotating-polarizer or rotating-analyzer ellipsometer, is termed"an incomplete" polarimeter, because it is insensitive to the handednessof the circularly polarized component and exhibits poor performance whenthe light being analyzed is either nearly completely linearly polarizedor possesses a depolarized component. However, using UV light fromsource 90, the substrate of materials such as silicon contribute enoughto the overall phase shift of the light interacting with the sample thataccurate results can be obtained without the use of a compensator. Insuch a case, the same formulas above can be used to derive thickness d,where the phase shift induced by the compensator is set to be zero.

It is to be understood that the present invention is not limited to theembodiments described above and illustrated herein, but encompasses anyand all variations falling within the scope of the appended claims. Forexample, beams 24, 26, and/or 106 can be transmitted through the sample,where the beam properties (including the beam polarization state) of thetransmitted beam are measured. Further, a second compensator can beadded, where the first compensator is located between the sample and theanalyzer, and the second compensator located between the sample and thelight source 90, as illustrated in FIG. 4. These compensators could bestatic or rotating. In addition, to provide a static or varyingretardation between the polarization states, compensator 98 can bereplaced by a non-rotating opto-electronic element or photo-elasticelement, such as a piezo-electric cell retarder which are commonly usedin the art to induce a sinusoidal or static phase retardation byapplying a varying or static voltage to the cell.

What is claimed is:
 1. An optical apparatus for evaluatingcharacteristics of a semiconductor test sample comprising:aspectroscopic measurement module including a broadband light source forcreating a probe beam directed to reflect off the test sample andincluding a detector for monitoring changes in either the polarizationor magnitude of the reflected probe beam at multiple wavelengths; aprocessor for evaluating the test sample based on the measured changesof the probe beam; and a calibration module including a reference sampleand an off-axis ellipsometer having a wavelength stable, narrowbandlight source for measuring the reference sample and wherein saidapparatus is arranged so that the reference sample is also measured bythe spectroscopic module and wherein the processor utilizes themeasurements of the reference sample by both the off-axis ellipsometerand the spectroscopic module to calibrate the spectroscopic module forsubsequent measurements of test samples.
 2. An apparatus as recited inclaim 1 wherein said spectroscopic module is a spectrophotometer whereinthe changes in magnitude of the reflected probe beam are measured at aplurality of wavelengths.
 3. An apparatus as recited in claim 1 whereinsaid spectroscopic module is a spectroscopic ellipsometer whereinchanges in the polarization state of the probe beam are analyzed at aplurality of wavelengths.
 4. An apparatus as recited in claim 1 whereinthe narrowband light source is defined by a gas discharge laser.
 5. Anapparatus as recited in claim 4 wherein said gas discharge laser is ahelium-neon laser.
 6. An apparatus as recited in claim 1 wherein saidnarrowband light source is defined by a laser diode.
 7. An apparatus asrecited in claim 1 wherein the narrowband wavelength source produceslight have a stable known wavelength to within one percent.
 8. Anapparatus as recited in claim 1 wherein the reference sample is definedby a substrate having an oxide layer thereon wherein the composition ofthe oxide is known prior to measurement while the thickness of the oxidelayer is unknown prior to measurement.
 9. A method of operating aspectroscopic apparatus to analyze the characteristics of asemiconductor test sample, wherein the spectroscopic apparatus includesa broadband light source for creating a probe beam directed to reflectoff the test sample and including a detector to monitor changes ineither the polarization or magnitude of the reflected probe beam at aplurality of wavelengths, said spectroscopic apparatus further includinga calibration module incorporating an off-axis ellipsometer having astable wavelength narrowband light source, said calibration modulefurther including a reference sample, said method comprising the stepsof:measuring the reference sample with the narrowband light source ofthe off-axis ellipsometer; measuring the reference sample with thebroadband light source of the spectroscopic apparatus; analyzing thecharacteristics of the reference sample using the measurements obtainedfrom both the off-axis ellipsometer and spectroscopic apparatus;comparing the analyses of the characteristics of the reference samplederived from measurements obtained from the off-axis ellipsometer andthe spectroscopic apparatus; calibrating the spectroscopic apparatusbased on the comparison of the analyses of the characteristics of thereference sample; and measuring and analyzing a test sample with thecalibrated spectroscopic apparatus.
 10. A method as recited in claim 9wherein the spectroscopic apparatus operates to measure changes in themagnitude of the reflected probe beam at a plurality of wavelengths. 11.A method as recited in claim 9 wherein the spectroscopic apparatusoperates to measure changes in the polarization state of the probe beamat a plurality of wavelengths.
 12. A method as recited in claim 9wherein the reference sample is defined by a substrate having an oxidelayer thereon.
 13. A method as recited in claim 12 wherein during saidstep of analyzing the reference sample, the thickness of the oxide layerof the reference sample determined based on the measurements made withthe spectroscopic apparatus is compared with the thickness of the oxidelayer determined based on the measurements made with the off-axisellipsometer in order to calibrate the spectroscopic apparatus.